1. Field of the Invention
This invention relates to a method and apparatus for flame gunning and more particularly to a method and apparatus for applying hot composite on top of the refractory lining of steel making and processing vessels, wherein said hot composite is periodically applied in order to maintain the desired shape of the refractory lining and to be consumed as a slag forming component during steel making and refining.
2. Description of the Prior Art
Flame gunning was first used with MgO powder for the sole purpose of hot refractory repair in the U.S.S.R. more than 15 years ago, primarily for basic oxygen furnaces (BOF). Later on, modifications of these flame gunning methods and apparatuses were made and used around the world. But in spite of such widely available knowledge about flame gunning, the practical use of this technology today is limited. Such limited use is a result of the additional expense and complexity of the flame gunning method of applying MgO powder in comparison with the relatively simple wet gunning of MgO powder, which provides similar longevity of the repaired refractory layer. In both cases, expensive MgO powder is applied only to restore the shape and thermal insulating characteristics of refractory lining and also for the prevention of refractory consumption by aggressive slag. To reduce the cost of the refractory lining, the use of burnt lime has been proposed in several Soviet Certificates of Inventions. Originally, the use of lime was proposed as a partial substitution for MgO (AC# 676579 U.S.S.R.). Later on, the use of lime with no MgO powder was suggested. In all of these cases, the use of solid carbonaceous, liquid or gaseous fuel and oxygen-rich oxidizing gas (typically pure oxygen) has been suggested to create a high temperature flame, which is used to heat refractory materials to a temperature exceeding the softening temperature of MgO positioned inside of the flame directed toward the refractory lining. When coke was added to the flame gunning mix as a fuel, a deposit layer having substantial porosity and containing unburned solid carbon was formed due to the inability of existing flame gunning systems to completely convert solid carbon into CO and CO.sub.2. The porosity of the deposit layer is caused by the oxidation of the deposited carbon to CO inside the deposit layer.
All of the above-described gunning mixtures were designed to provide conversion of MgO and dolomite clinker into plastic form inside of the flame envelope directed toward the hot refractory wall. Conversion of these refractory materials into plastic form at high temperature is required in order to produce a deposit layer of gunned material by striking this material against the wall. No requirement for any additional binding components has been suggested or identified in the above Certificates of Inventions.
Various designs of flame gunning apparatus were proposed to provide for the use of gaseous fuel, for example natural gas, to avoid depositing solid carbon in the layer to reduce its porosity. Unfortunately, the use of gaseous fuel results in reduction of the rate of heat transfer from the flame to the refractory wall, leading to rapid cooling of the first portion of the deposit layer contacting the relatively cold refractory wall surface. Rapid cooling diminishes the adhesive strength between the refractory lining and the gunned layer.
The main purpose of the gunning apparatus using the above-described gunning mix (containing mainly MgO powder) was to apply this mix in such a way that mechanical binding between the hot, softened plastic powder and the rough surface of the refractory wall would allow patching of local wear of refractory walls and to prolong the life of the furnaces without stopping the operation for refractory brick relining.
Later, the idea to use lime as a refractory component of the gunning mix arose from the well known practice of applying slag-forming materials to protect refractory walls without flame gunning. (O. N. Chemesis et al. Chernaya Metallurgia, Bulletin, ITI N22:51-52, 1974). Substitution of MgO based flame gunning material with slag forming lime-based flame gunning material was suggested to reduce the cost of the gunning mixture, but was not successfully implemented due to the low longevity of gunned lime-based deposits. This low longevity is caused by insufficient binding strength. The use of more than 5, but less than 10% of blast furnace slag containing 35-40% of SiO.sub.2 (i.e., a total content of between 1.75% and 4% SiO.sub.2) has been suggested (A.C. #935497, U.S.S.R.) to provide a fused silica-based, low melting and fluidizing temperature binding additive to the flame gunning mixture to be used with conventional flame gunning machines. The use of 5% or less of this binding additive (providing 1.75% to 2% SiO.sub.2 by weight of the gunning mixture) was considered undesirable due to diminished binding strength of a gunned deposit having an insufficient presence of SiO.sub.2. The use of more than 10% of such binding additive (providing more than 3.5 to 4% SiO.sub.2 of the gunning mixture) was also considered undesirable due to an excessive reduction in the melting temperature of the gunned deposit.
Existing flame gunning apparatus designs cannot simultaneously provide sufficient kinetic energy and temperature for the hot gunned mixture impacting the refractory lining. This prevents a successful utilization of the above-suggested flame gunning mixture and leads to as high as a 5-10% carry-over of unbound lime dust into the air pollution control system, thereby rapidly diminishing its performance. The complete surrounding of the carrier gas stream with the oxidizer stream in existing flame gunning apparatuses helps to reduce loss of the gunning mixture from the flame but leads to delayed ignition of the oxygen and carbonaceous fuel mixture. This delayed ignition results in a reduced time available for heating of the flame gunning mixture and for oxidation of solid carbonaceous fuel inside the flame. This insufficient oxidation of carbonaceous fuel together with the limited heat transfer inside the flame envelope causes a continuous presence of substantial amounts of unburned carbon inside the deposit layer. This leads to formation of a highly porous gunned layer because of the subsequent oxidation of the deposited carbon into CO gas.
Thus, existing pyroplastic flame gunning technologies limit the melting of silica based binding additives inside the flame envelope during the flame gunning process, limiting the reaction of fused silica based components with lime particles inside of the flame envelope, which, in turn, limits the total amount of binding additives that can be used without causing excessive fluidity of the deposit layer due to excessive presence of undissolved fused silica in this layer. This necessity to limit the amount of silica based components and the insufficient high-temperature reaction time available inside the flame results in reduced density of the initially formed intermediate or transitional layer, which is the layer responsible for the adhesive strength between the main gunned deposit and the refractory lining. The substantial presence of unburned solid carbon in the intermediate deposit layer further reduces adhesive strength due to an increase in the porosity of the deposit layer. Insufficient velocity and kinetic energy of the hot gunning material impacting the refractor wall also contributes to the high porosity of the intermediate layer produced by conventional flame gunning apparatuses.